Wireless Communications System and Method

ABSTRACT

The Invention is directed to a mobile communications system having improved spectral efficiency. The invention is further directed to methods and apparatus to achieve this improved spectral efficiency. In the mobile communications system communication with a plurality of mobile terminals is provided by a base station. Within the system terminals are adapted to communicate with one or more adjacent similar terminals to establish groups of terminals, called micro-cells. Each terminal within a micro-cell receives signals from the base station and then performs a first processing step on these signals. These processed signals are shared with all the other terminals within the micro-cell. Each terminal then performs a second processing step on the information it has received from all the other terminals within the micro-cell which enables it to derive a signal intended for reception by that terminal. The technique is applicable to both the uplink (user to base station) and the down link (base station to user) and also to peer to peer (user to user) communication.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 13/347,879, filed Jan. 11, 2012, entitled “WirelessCommunications System and Method”, invented by James Naden et al., whichis a continuation of U.S. patent application Ser. No. 10/261,068, filedSep. 30, 2002 (now U.S. Pat. No. 8,204,504), which claims the benefit ofU.S. Provisional Application No. 60/337,547, filed Oct. 26, 2001. All ofthe above-identified Applications are hereby incorporated by referencein their entireties as though fully and completely set forth herein.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for improving thespectral efficiency of mobile cellular communications systems. Theinvention also relates to a mobile communications system having suchimproved spectral efficiency.

BACKGROUND TO THE INVENTION

Mobile cellular operators are placing increasing demands on capacity inorder to support greater numbers of subscribers and higher bit-rateservices. This in turn is placing increasing pressure on the restrictedamount of available radio spectrum. In attempts to provide moreefficient use of the available spectrum, workers in this field haveconsidered the use of spatial processing and of the provision ofmicro-cells.

Spatial processing exploits the multi-path characteristics of the mobileradio channel by means of multiple antennas at the transmitter and atthe receiver. The benefit arises in two ways. Firstly, there is adiversity gain, which arises provided the antennas at the receiver aresufficiently far apart for the signals they receive to be uncorrelated.Then, the signal received at any one antenna varies independently of thesignals received by the other antennas and the signal-to-noise ratio ofthe combined signal is improved as a consequence. Secondly, themulti-path channel can be separated into independent spatial modes, eachof which is capable of supporting traffic in its own right. To exploitthis effect requires coding of the signal at the transmitter andmultiple transmit antennas. The combination of these two effects,diversity gain and independent spatial modes, leads to an improvement inspectral efficiency that is proportional to the product of the number oftransmit antennas and the number of receive antennas. Use of either ofthese effects alone may still provide a useful improvement, albeitsmaller than that realised in combination. The improvement in spectralefficiency is only realised if the antennas are sufficiently far apartfor the received signals to be uncorrelated. In practice this means thatthe antennas must be separated by at least one half wavelength at thefrequency of the radio signal, thus restricting the application ofspatial processing to higher frequencies and or larger antennainstallations. Recent work has sought to overcome this restriction bysuggesting that the antennas need not be placed on the same terminal,either at the transmitter or at the receiver, and that groups ofterminals can cooperate to form virtual transmit and receive antennaarrays.

Micro-cells are an extension of the cellular concept to smaller cells inan attempt to accommodate larger numbers of users. In cellular radio,the radio spectrum that is used in one geographical area, or cell, maybe re-used in other cells, provided that the cells are sufficiently farapart for mutual interference to be below a pre-determined level. Thelevel of interference is dependent on the ratio of the cell diameter andthe distance between cells in such a way that it remains constant ifboth are changed in proportion, for example if both are halved. Hencehigher capacity density can be achieved by means of smaller cells. Thesmall cells typical of micro-cellular architectures therefore offer thepotential for very high capacity density and hence high overall spectralefficiency. However, such architectures, while offering high capacitydensity within the cells, pose the difficult backhaul problem of linkingthe myriad cells back into the network. There is also the requirementfor an added layer of wireless infrastructure. The use of micro-cellshas therefore been usually restricted to areas where an optical fibreinfrastructure is readily accessible, such as in-building.

OBJECT TO THE INVENTION

The invention seeks to provide a wireless communications system andmethod which mitigates at least one of the problems of known methods.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a mobilecommunications system in which communication with a plurality of mobileterminals is provided by a base station, wherein said terminals areadapted each to communicate with one or more adjacent similar terminalsto establish disjoint groups of terminals, each said group functioningco-operatively as a micro-cell for communications purposes, wherein eachterminal in a said group is arranged to perform a first processing stepon signals received from the base station, to exchange with the otherterminals of the group information derived from said first processingstep, and to perform a second processing step utilising exchangedinformation received from other terminals of the group so as to derive,from the received signals, a signal intended for reception by thatterminal.

The number of terminals within a group is such that the spectralefficiency of the system is maximised.

Preferably, the amount of processing that is performed in the firstprocessing step is controlled so as to enhance the spectral efficiencygain.

The base station may be arranged to transmit simultaneously a pluralityof signals one for each mobile terminal of the group.

Each said transmitted signal may be encoded with a respective uniquespreading sequence, and said spreading sequence may comprise a Walshcode.

The system may comprise a code division multiple access (CDMA) system.

The terminals may be selected to form part of said group by monitoringthe strength of a received signal from the base station and selectingterminals with the highest received signal strength.

According to another aspect of the invention there is provided a mobilecommunications system in which a plurality of mobile terminalscommunicate with one another in a peer to peer manner, wherein saidterminals are adapted each to communicate with one or more adjacentsimilar terminals to establish groups of terminals, each said groupfunctioning co-operatively as a micro-cell for communications purposes,wherein each terminal in a said group is arranged to perform a firstprocessing step on signals received from outside the group, to exchangewith the other terminals of the group information derived from saidfirst processing step, and to perform a second processing step utilisingexchanged information received from other terminals of the group so asto derive, from the received signals, a signal intended for reception bythat terminal.

Advantageously, spectrum is reserved for intra-group communication so asto enhance spectral efficiency.

In a preferred arrangement, the groups of mobile terminals are formed onan ad-hoc basis and comprise a number of terminals such that the numberof terminal antennas being serviced by the base station is as close aspossible to a predetermined optimum number. Each base station antennatransmits to all terminals of the group a respective signal intended forreception by one terminal of that group. Each terminal processes thereceived signals to determine channel estimates which are then used in amatched filter to reduce channel distortion prior to the correlationprocess. This information is shared by the terminals to facilitaterecovery of the signals intended for each terminal of the group.

Advantageously, each signal transmitted by the base station is encodedwith a respective unique spreading sequence, e.g. a Walsh code. Pilotsequences may be added to the transmitted signals to facilitate channelestimation.

According to another aspect of the invention there is provided a methodof providing spectrum re-use in a mobile communications system in whichcommunication with a plurality of mobile terminals is provided by a basestation, and wherein said terminals are adapted each to communicate withone or more adjacent similar terminals to establish groups of terminals,each said group functioning co-operatively as a micro-cell forcommunications purposes, the method comprising: transmitting signalsfrom the base station to all terminals of the group; at each terminal ofsaid group, performing a first processing step on signals received fromthe base station; exchanging with the other terminals of the groupinformation derived from said first processing step, and performing asecond processing step utilising exchanged information received fromother terminals of the group so as to derive, from the received signals,a signal intended for reception by that terminal, and wherein the numberof terminals within a said group is selected such as to maximise thespectral efficiency of the system.

Preferably, the amount of processing that is performed in the firstprocessing step is controlled so as to enhance the spectral efficiencygain.

Preferably, the base station transmits simultaneously a plurality ofsignals one for each mobile terminal of the group.

Each said transmitted signal may be encoded with a respective uniquespreading sequence and each said spreading sequence may comprise a Walshcode.

The system may comprise a code division multiple access (CDMA) system.

The terminals may be selected to form part of said group by monitoringthe strength of a received signal from the base station and selectingterminals with the highest received signal strength.

According to another aspect of the invention there is provided a methodof providing spectrum re-use in a mobile communications system in whicha plurality of mobile terminals communicate with one another in a peerto peer manner, and wherein said terminals are adapted each tocommunicate with one or more adjacent similar terminals to establishgroups of terminals, each said group functioning co-operatively as amicro-cell for communications purposes, the method comprising: receivingsignals at all terminals of the group; at each terminal of said group,performing a first processing step on the received signals; exchangingwith the other terminals of the group information derived from saidfirst processing step, and performing a second processing step utilisingexchanged information received from other terminals of the group so asto derive, from the received signals, a signal intended for reception bythat terminal, and wherein the number of terminals within a said groupis selected such as to maximise the spectral efficiency of the system.

The method may be performed by software in machine readable form on astorage medium.

According to another aspect of the invention there is provided a mobileterminal for use in a mobile communications system in whichcommunication with a plurality of similar mobile terminals is providedby a base station, wherein said terminal is adapted to communicate withone or more adjacent similar terminals to establish a group of terminalsfunctioning co-operatively as a micro-cell for communications purposes,and wherein said terminal is arranged to perform a first processing stepon signals received from the base station, to exchange with the otherterminals of the group information derived from said first processingstep, and to perform a second processing step utilising exchangedinformation received from other terminals of the group so as to derive,from the received signals, a signal intended for reception by thatterminal.

The mobile terminal may select which adjacent similar terminals willco-operate as a micro-cell by receiving signals from adjacent terminals,wherein each terminal monitors the strength of the signal it receivesfrom the base station, and selecting terminals with the highest receivedsignal strength.

According to another aspect of the invention there is provided a mobileterminal for use in a mobile communications system in which a pluralityof mobile terminals communicate with one another in a peer to peermanner, wherein said terminal is adapted to communicate with one or moreadjacent similar terminals to establish a group of terminals functioningco-operatively as a micro-cell for communications purposes, and whereinsaid terminal is arranged to perform a first processing step on receivedsignals, to exchange with the other terminals of the group informationderived from said first processing step, and to perform a secondprocessing step utilising exchanged information received from otherterminals of the group so as to derive, from the received signals, asignal intended for reception by that terminal.

According to another aspect of the invention there is provided a methodof forming a micro-cell in a mobile communications system, saidmicro-cell comprising at least two mobile terminals adapted tocommunicate with one another in a peer to peer manner, the methodcomprising: receiving signals at a terminal from a plurality of adjacentsimilar terminals; each said terminal monitoring the strength of asignal received from the base station and selecting terminals with thehighest received signal strength to operate within said micro-cell.

In our technique we exploit the advantages of space-time processing andmicro-cells in such a way as to overcome the disadvantages of both, thusproviding enhanced spectral efficiency in such a way that it isaccessible with small terminals and low frequencies. The technique isapplicable to both the uplink (user to base station) and the down link(base station to user) and also to peer to peer (user to user)communication.

The preferred features may be combined as appropriate, as would beapparent to a skilled person, and may be combined with any of theaspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described with reference tothe accompanying drawings in which:

FIG. 1 is a schematic diagram of part of a mobile communications systemaccording to a preferred embodiment of the invention;

FIG. 2 illustrates in simplified schematic form the communicationprocess between a base station and a group of mobile terminals in thesystem of FIG. 1;

FIG. 3 illustrates the process of signal reception and a first stage ofsignal processing in one of the user terminals of the group of mobileterminals of FIG. 2;

FIG. 4 illustrates a second stage of signal processing in one of theuser terminals of the group of mobile terminals of FIG. 2;

FIG. 5 illustrates the process of signal transmission from the group ofmobile terminals of FIG. 2;

FIG. 6 illustrates graphically a first example of the form of thespectral efficiency improvement that may be obtained by the system ofFIG. 1.

FIG. 7 illustrates graphically a second example of the form of thespectral efficiency improvement that may be obtained by the system ofFIG. 1.

FIG. 8 illustrates graphically a third example of the form of thespectral efficiency improvement that may be obtained by the system ofFIG. 1.

FIG. 9 illustrates graphically a fourth example of the form of thespectral efficiency improvement that may be obtained by the system ofFIG. 1.

FIG. 10 illustrates graphically a fifth example of the form of thespectral efficiency improvement that may be obtained by the system ofFIG. 1.

FIG. 11 illustrates graphically a sixth example of the form of thespectral efficiency improvement that may be obtained by the system ofFIG. 1.

DETAILED DESCRIPTION OF INVENTION

The application of our technique in a mobile cellular radio environmentis shown diagrammatically in FIG. 1 and consists of a macro-cellulararchitecture overlaid on a pseudo micro-cellular architecture as will bedescribed below.

The cellular architecture shown in FIG. 1 will typically comprise a CDMA(Code Division Multiple Access) system using spatial processing.Although this is our preferred embodiment, it will be apparent to thoseskilled in the art that the methods described below are also applicableto other multiple access techniques such as TDMA (Time Division MultipleAccess).

As indicated in FIG. 1, each macro-cell 11 is centred on a base station12. The base station is provisioned with multiple antennas 16, althoughit is also envisaged that a group of base stations, each equipped with asingle antenna, could cooperate to similar effect. Spatial processing isemployed within the macro-cell 11, providing spectrally efficient linksbetween the terminals in individual pseudo micro-cells or groups 15 andthe base station, at which point access to the fixed network (not shown)is provided. Consequently there is no need for a fixed infrastructure tolink the micro-cells to the network.

The user terminals 13 are assumed to be mobile. Each user terminal isprovisioned with an antenna 17 for communication with the base station12, although it is straightforward to generalise this concept to includeuser terminals with more than one antenna. To obtain the gain fromspatial processing, which requires multiple antennas, each user terminal13 cooperates with a number of other conveniently located user terminalsin its vicinity by forming an ad-hoc network comprising a pseudomicro-cell. These groupings constitute the pseudo micro-cells orterminal groups 15 of the architecture shown in FIG. 1 and change on anad-hoc basis as user terminals move within the macro cell 11 or to andfrom adjacent macro cells (not shown). Thus, as the user terminals roam,existing groups are dissolved and new groups are formed. Thedetermination of a preferred number of terminals to form a group will bediscussed below. Intra-group communications (also calledintra-micro-cell or intra-cell or inter-terminal communications becauseit is between terminals in a group) are carried within a reservedspectrum f₁, this spectrum being allocated for use by all ad-hoc groupsof terminals within the macro-cell 11 and within all other macro-cellsof the system. Not all of the reserved spectrum f₁ need necessarily beused within every micro-cell but all intra-group (or inter-terminal)communication will occur within this reserved spectrum.

As the antennas 17 forming the link with the base station 12 are onseparate user terminals, there is a low correlation between theirrespective signals, even at low frequencies, and there is no constrainton user terminal size caused by the need to keep adjacent antennas atleast half a wavelength apart. Furthermore, the reduced number ofantennas on a user terminal and the lower number of RF receive chainsrequired as a consequence results in considerable savings in userterminal cost and complexity.

It will be appreciated that, in an alternative environment where directpeer to peer communication is the dominant mode of operation, a centralbase-station does not normally exist, although a larger platform, suchas a vehicle, may behave similarly. Alternatively, both ends of the linkmay terminate on ad-hoc networks in pseudo micro-cells.

Within the pseudo micro-cell or terminal group 15, the frequencies orCDMA codes utilised are selected from a different group from those usedbetween the base station and the user terminals, or between userterminal groups in peer to peer communications, and conform to adifferent frequency re-use pattern. The re-use pattern used by thepseudo micro-cells is independent of that used by the macro-cells. Thusany spectrum used in the micro-cells can be re-used in the micro-cellsof every macro-cell, irrespective of the macro-cell re-use factor.

The terminals 13 exchange the necessary information within their ownpseudo micro-cell or group to decode the space-time signals from thebase station and to encode the space-time signals for the base station.This exchange of information within the terminal group 15 may occur aspart of an integrated protocol for the entire system or alternativelymay make use of an existing protocol, such as a wireless local areanetwork, for example IEEE 802.11. Protocols supporting ad-hoc networkingare particularly suitable, for example Bluetooth. The amount ofinformation to be exchanged is relatively high and without the frequencyre-use afforded by the micro-cells would more than offset the gainprovided by the spatial processing. Including this frequency re-use,however, gives rise to a net gain in the spectral efficiency of thetotal system compared to that of the macro-cell alone.

The way in which the information is exchanged in the system of FIG. 1influences the overall spectral efficiency significantly. For simplicityof explanation, the information exchange process is described below withreference to FIG. 2 for a base station having two antennas communicatingwith two user terminals, but it will be appreciated that the concept canbe readily extended to systems with additional base station antennas andlarger groups of user terminals or to systems in which both ends of thelink terminate on a group of user terminals in a pseudo micro-cell.

In FIG. 2, each terminal 13A, 13B is shown as having a primary antenna17A, 17B for communication with the base station and a secondary antenna171A, 171B for communication with other terminals of the group. Thissecondary antenna may be a discrete component as shown in FIG. 2, or itmay be incorporated in the main antenna. Thus, the main antenna may havea dual role.

As shown in FIG. 2, signal S₁ is transmitted from the base station 12 touser terminal 13A and signal S₂ is transmitted from the base station 12to user terminal 13B. Signal S₁ is encoded with a first unique spreadingsequence W₄, e.g. a Walsh code, and comprises a sequence of space-timesymbols which are transmitted from the base station 12 via antennas 16Aand 16B. Similarly, signal S₂ is encoded with a second unique spreadingsequence W₅ and is also transmitted from the base station 12 viaantennas 16A and 16B as a sequence of space-time symbols. In addition, afirst pilot sequence P₁ is transmitted from antenna 16A of the basestation and a second pilot sequence P₂ is transmitted from antenna 168of the base station. These transmissions from the base station 12 arewithin the spectrum allocated to the macro-cell. Within the ad-hocterminal group or micro-cell, all of the transmissions from the basestation 12 are received by both user terminal 13A via antenna 17A anduser terminal 13B via antenna 17B.

The radio channel between base station antenna 16A and antenna 17A onuser terminal 13A is denoted in FIG. 2 as h₁₁. Similarly, the radiochannel between base station antenna 16A and antenna 17B on userterminal 13B is denoted in FIG. 2 as h₁₂, the radio channel between basestation antenna 16B and antenna 17A on user terminal 13A is denoted inFIG. 2 as h₂₁, and the radio channel between base station antenna 16Band antenna 17B on user terminal 13B is denoted in FIG. 2 as h₂₂.

In order to extract the signal S₁ from the received signals, userterminal 13A requires knowledge of the signals received by user terminal13B and of the channels h₁₁ and h₂₁. Similarly, in order to extract thesignal S₂ from the received signals, user terminal 13B requiresknowledge of the signals received by user terminal 13A and of thechannels h₁₂ and h₂₂.

This information is obtained in a digital implementation of a CDMAsystem by the method illustrated in FIG. 3. As can be seen from FIG. 3,the digitised base-band signal in user terminal 13A is passed via aninput stage or front end shown schematically as comprising amplifier301, frequency converter 302 and analogue to digital converter (ADC) 303to a bank of channel estimators 31A, 31B, one for each base stationtransmit antenna 16A, 16B. Again, for simplicity only two channelestimators are shown although it will of course be appreciated that alarger number of base station antennas and channel estimators may beprovided. The first channel estimator 31A estimates the channel h₁₁ frombase station antenna 16A (FIG. 2) to antenna 17A on user terminal 13Ausing the known pilot sequence P₁. This first channel estimate is usedto form the matched filter 32A for channel h₁₁ through which the signalis then passed. Similarly, the second channel estimator 31B estimatesthe channel h₂₁ from base station antenna 16B (FIG. 2) to antenna 17A onuser terminal 13A using the known pilot sequence P₂. This second channelestimate is used to form the matched filter 32B for channel h₂₁ throughwhich the signal is then passed. The filtered signals are then passed torespective parallel banks of code correlators 33A, 33B. Each bank ofcode correlators has one correlator for each of the unique spreadingsequences W₄ and W₅. The correlators split the signal according to theunique spreading sequence and remove the unique spreading sequences.

An analogous process takes place in terminal 13B and any other member ofthe terminal group. The digitised base-band signal in user terminal 13Bis similarly passed to a bank of channel estimators, one for each basestation transmit antenna. The first channel estimator estimates thechannel h₁₂ from base station antenna 16A to antenna 17B on userterminal 13B using the known pilot sequence P₁. This channel estimate isused to form the matched filter for channel h₁₂ through which the signalis then passed. Similarly, the second channel estimator estimates thechannel h₂₂ from base station antenna 16B to antenna 17B on userterminal 13B using the known pilot sequence P₂. This channel estimate isused to form the matched filter for channel h₂₂ through which the signalis then passed. The filtered signals are then passed to parallel banksof code correlators. Each bank of code correlators has one correlatorfor reach of the unique spreading sequences W₄ and W₆. The correlatorssplit the signal according to the unique spreading sequence and removethe unique spreading sequences.

At this stage the intermediate signals s_(a), s_(b), s_(c), s_(d) outputfrom the code correlators are still in soft form: that is, eachintermediate signal is a digital representation of an analogue signalrather than a digital binary signal.

Having determined the signal estimates, user terminals 13A and 13B thenexchange their signal estimates so that both terminals now have allavailable information concerning the transmitted signals S₁ and S₂. Theextent to which the signals are processed prior to being transmitted tothe other user terminals in the group is chosen so that the amount ofinformation to be exchanged is minimised consistent with minimising theprobability of error in the final output following further processing ofthe signals as will be described below.

The necessary information is exchanged between the user terminals usingthe antennas 171A and 171B (denoted WLAN) in FIG. 2. These antennas areshown separately for clarity. However, as discussed above, separateantennas are not essential and antenna 17A on user terminal 13A andantenna 17B on user terminal 13B may be used instead. Transmissionsbetween the terminals are within the spectrum f₁ allocated to the pseudomicro-cell. Intra-micro-cell communication within the pseudo micro-cellmay be digital or analogue.

Having obtained the information from the other user terminals in thegroup, namely user terminal 1313 in this example, user terminal 13A isnow able to perform further processing of the intermediate signalss_(a), s_(b), s_(c), s_(d). A preferred embodiment of this furtherprocessing is shown in FIG. 4. The initial stage employs a multi-userdetection algorithm 41 which reduces the residual mutual interferencebetween signals from different sources or intended for differentterminals that has not been removed by the correlators. Demodulation ofthe signals is then performed using a combination of linear MMSE 42 andViterbi 43 sequence estimators. Having estimated the signals S₁ and S₂,the corresponding intermediate signals are then reconstructed 44 as theywould appear at the input and, by comparison with the actualintermediate signals, any residual interference is estimated.Subtracting this interference from the input 45 and recalculating thesignals S₁ and S₂ then improves the error performance.

Analogous processing also takes place in user terminal 13B. In a similarmanner, the initial stage of terminal 13B employs the multi-userdetection algorithm which reduces the residual mutual interferencebetween signals from different sources or intended for differentterminals that has not been removed by the correlators. Demodulation ofthe signals is then performed by sequence estimation using a combinationof linear and Viterbi sequence estimators. Having estimated the signalsS₁ and S₂, the corresponding intermediate signals are then reconstructedas they would appear at the input and, by comparison with the actualintermediate signals, the interference is estimated. Subtracting thisinterference from the input and recalculating the signals S₁ and S₂ thenimproves the error performance.

It will be apparent that both signals S₁ and S₂ are available in bothuser terminal 132A and user terminal 13B. Consequently, where allowedfor in a higher layer protocol, such as ATM or TCP/IP, statisticalmultiplexing gain may be realised in addition to the spatial processinggain previously described. Statistical multiplexing gain arises becauseneither user terminal is restricted to the capacity provided by S₁ or S₂but may, at any one instant, make use of a capacity up to that of thecombined capacity of the user terminals in the group, provided it is notbeing used by other user terminals. This is particularly advantageouswhere variable rate traffic is predominant and the instantaneouscapacity demanded by a terminal fluctuates.

A further embodiment of the invention is illustrated in FIG. 5 whichshows an alternative communication protocol between the terminals andthe base station. In the uplink direction, from user terminal 13A, 13Bto base station 12, or in peer to peer communication where thetransmitter is also a user terminal in a pseudo micro-cell, thetransmission process is slightly modified from that described above withreference to FIG. 2 for a base station with multiple antennas. As shownin FIG. 5, signal S₃ is transmitted from user terminal 13A to the basestation 12, and signal S₄ is transmitted from user terminal 13B to thebase station 12. Signal S₃ and signal S₄ each comprise of a sequence ofspace-time symbols. These signals are exchanged between user terminal13A and user terminal 13B so that both terminals have knowledge of bothsignals. The necessary information is exchanged between the userterminals using the antennas 171A, 171B (denoted WLAN in FIG. 5).Although these antennas 171A and 171B are shown separately for clarity;it will be understood from the description with reference to FIG. 2above that separate antennas are not essential. Transmissions betweenthe terminals 13A and 13B are within the spectrum allocated to thepseudo micro-cell. By exchanging the signals prior to introducing thespreading sequences, the amount of data to be exchanged is minimised.Signal S₃ is encoded with a first unique spreading sequence W₆, e.g. aWalsh code, and is transmitted to the base station via antenna 17A ofuser terminal 13A and antenna 17B of user terminal 13B. Similarly,signal S₄ is encoded with a second unique spreading sequence W₇ and isalso transmitted to the base station via antenna 17A of user terminal13A and antenna 17B of user terminal 13B. In addition, a first pilotsequence P₃ is transmitted from antenna 17A of user terminal 13A and asecond pilot sequence P₄ is transmitted from antenna 17B of userterminal 17B.

The benefit in terms of improved spectral efficiency can be estimatedmathematically as follows.

Let us assume that the total available spectrum is S and that thespectrum allocated to the micro-cells is φω, where ω is the totalspectrum available for intra-communication within the group ofcooperating terminals, that is within a single micro-cell, and φ is there-use factor applying to this frequency. If the macro-cellularfrequency re-use factor is ρ then the spectrum available in eachmacro-cell is (S−φω)/ρ. In a conventional system not employing thismethod, the spectrum available in each macro-cell is simply S/ρ. Thespectral efficiency improvement is therefore given by

ξ=(1−φω/S)·G _(sp) G _(sm)  Equation 1:

where G_(sp) is the spatial processing gain and G_(sm), is thestatistical multiplexing gain.

The value of G_(sp) will depend upon the characteristics of the radiochannel and on the numbers of antennas at the receiver and at thetransmitter. Consider, for example, a symmetric system in which m, thenumber of transmit antennas, is equal to n, the number of receiveantennas. It is well known in the literature that G_(sp) isapproximately equal to m in such a symmetric system. In the case wherethe traffic is constant bit rate, such that G_(sm) is unity, thespectral efficiency improvement is then given by

ξ=(1−φω/S)·m  Equation 2

We make use of Shannon's capacity formula to estimate the correspondingcapacity C_(m,n) that can be supported in a given channel bandwidth W ata given signal to noise ratio SNR_(1,1), as follows:

$\begin{matrix}{C_{m,n} = {m\; {{W\left( {1 - \frac{\omega \; \phi}{S}} \right)} \cdot {\log_{2}\left( {1 + {n \cdot \frac{{SNR}_{1,1}}{m}}} \right)}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

In equation 3 we have again assumed that G_(sm) is equal to one, and thereduction in effective channel bandwidth is accounted for by the term(1−exp/S).

It then remains to determine the proportion of spectrum that must be setaside for intra-cell communication, that is between cooperating mobileterminals in a micro-cell. This will depend upon whether analogue ordigital means are employed for intra-cell communication, whether it isthe transmitter or the receiver or both that is distributed in themicro-cell, whether it is a downlink or an uplink, and whether thecooperating terminals are themselves also actively engaged in their ownsessions or are otherwise idle.

A single example is included here and is the case corresponding to thedownlink, in which a base station with multiple antennas is transmittingto a first mobile terminal, which is in a micro-cell with other similarterminals. The other terminals in the micro-cell are assumed to besimilarly engaged in their own sessions, in addition to the signalsrequired to be received and transmitted in respect of the first mobileterminal. Intra-cell communication is assumed to be digital. A number ofdifferent examples are included in appendix 1 and FIGS. 7-11.

In the downlink example described above the receiver is distributed,such that several terminals, each equipped with a single antenna,cooperate to enhance the signal received from a multi-antenna basestation. In this scenario, some spectrum is required for communicationbetween the cooperating terminals. Let the transmitter of the basestation be denoted transmitter T and have m transmit antennas and thefirst mobile terminal be denoted receiver R and have a single receiverantenna. Let there be n terminals forming the micro-cell, includingreceiver R.

For each bit of a signal from transmitter T to receiver R, 1/mspace-time symbols are transmitted. At each of the n receivers in thecooperating group, these space-time symbols must each be resolved to adepth of log₂(k·SNR) bits, where k is a constant greater than unity, ifquantisation noise is to be small in relation to other noise in thesystem. Each receiving antenna, with the exception of that on receiverR, for which the signal is ultimately intended, must transmit itsversion of the received signal to receiver R. The total number of bitsthat must be transmitted by the (n−1) receivers to receiver R istherefore

$\begin{matrix}{\frac{n - 1}{m} \cdot {\log_{2}\left( {k \cdot {SNR}_{1,1}} \right)} \cdot C_{m,n}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

Similarly, each receiver must make an estimate of each of the m channelsbetween its antenna and the m transmitting antennas. These channelestimates must be to a resolution comparable with that of the signal:log₂(k·SNR) bits. Each of the receiving antennas, other than receiver R,must pass this information to receiver R. However, because the channelschange more slowly than the bit rate of the signal, channel updateinformation need only be transmitted once per frame; let the frame sizebe F bits. Furthermore, the channel information is common to all of then signals transmitted by the transmitter to the n terminals in thecooperating group, so only a fraction 1/n of the channel updateinformation need be allocated to each signal.

Note: it is an assumption that the receiver R cooperates with otherterminals engaged in their own sessions and not with idle terminals. Ifthe latter is the case, then the factor 1/n does not apply and this termis proportionately larger. Such a situation is described in Appendix 1.

Hence, the proportion of the channel update information transmitted bythe (n−1) receiving antennas to receiver R corresponding to the signalintended for receiver R is

$\begin{matrix}{\frac{\left( {n - 1} \right)}{n} \cdot \frac{m}{F} \cdot {\log_{2}\left( {k \cdot {SNR}_{1,1}} \right)} \cdot C_{m,n}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

Combining this with the signal information and setting m=n we obtain theproportion of the total information that must be transmitted between thereceiving terminals corresponding to the signal intended for receiver R.

$\begin{matrix}{\left( {\frac{1}{m} + \frac{1}{F}} \right){\left( {m - 1} \right) \cdot {\log_{2}\left( {k \cdot {SNR}_{1,1}} \right)} \cdot C_{m,n}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

We again make use of Shannon capacity formula and solve for the ratioωφ/S as follows:

$\begin{matrix}{{\left( {\frac{1}{m} + \frac{1}{F}} \right){\left( {m - 1} \right) \cdot {\log_{2}\left( {k \cdot {SNR}_{1,1}} \right)} \cdot C_{m,n}}} = {\frac{\omega}{m} \cdot {\log_{2}\left( {1 + {SNR}_{1,1}} \right)}}} & {{Equation}\mspace{14mu} 7}\end{matrix}$

Note the factor 1/m on the RHS of equation 7, which arises because ωrepresents the total available spectrum for intra-communication withinthe group for all n signals destined for the group. Recalling that wehave set m=n, the proportion required for the signal intended forreceiver R is therefore ω/m.

Substituting for C_(m,n) from equation 3, setting m=n, and solving forthe ratio of the spectrum required for intra-communication within thegroups ωφ to the total spectrum S, we obtain:

$\begin{matrix}{{\frac{\omega \; \phi}{S} - {1/1} + \alpha}{where}} & {{Equation}\mspace{14mu} 8} \\{\alpha = {{1/\left( {1 + \frac{m}{F}} \right)}{{m\left( {m - 1} \right)} \cdot {\log_{2}\left( {k \cdot {SNR}_{1,1}} \right)}}\frac{\omega \; \phi}{S}}} & {{Equation}\mspace{14mu} 9}\end{matrix}$

The relative capacity for a distributed system with m=n thus becomes

$\begin{matrix}{\mspace{79mu} {{C_{m,n}/C_{1,1}} = {m\left( {1 - \frac{1}{1 + \alpha}} \right)}}} & {{Equation}\mspace{14mu} 10} \\{{C_{m,n}/C_{1,1}} = {m\left( \frac{1}{1 + {\left( {1 + \frac{m}{F}} \right){{m\left( {m - 1} \right)} \cdot {\log_{2}\left( {k \cdot {SNR}_{1,1}} \right)}}\frac{W\; \phi}{S}}} \right)}} & {{Equation}\mspace{14mu} 11}\end{matrix}$

From equation 11 we can see that a narrowband signal (W), a largespectral allocation (S) and a low frequency re-use factor (φ) are keyenablers of high spectral efficiency in a distributed receiver system.

This is represented graphically in FIG. 6, which represents a systemwith a 10 kHz data channel to each terminal and other parameters asshown in the table below.

Line F k SNR W φ S 61 100 10 10 10 4 1250 62 100 10 10 10 4 5000 63 100010 10 10 4 5000 64 1000 4 10 10 4 5000 65 1000 4 5 10 4 5000 66 1000 4 510 1 5000 67 Equivalent performance for a standard MIMO (Multiple InputMultiple Output) system

FIG. 6 shows that the overall capacity gain is relatively insensitive tothe frame size (F) and the signal to noise ratio (SNR) and is dominatedby the frequency re-use (φ) and spectral allocation (S). At the peakcapacity gain of between 2 and 3, approximately 50% of the overallspectrum of for example 5 MHz is required for communication within theterminal groups, for which purpose WLANs of nearly 1 MHz bandwidth arerequired. The system is therefore suited to low data-rate terminals, andan efficient ad-hoc network protocol will be required to form spectrallyefficient terminal groups.

It will be apparent to those skilled in the art that the method followedin the example above can be readily adapted to similarly determine theproportion of spectrum that must be set aside for intra-cellcommunication (where this is communication within the micro-cell) in theother cases, for example where analogue means are employed forintra-cell communication, or where the transmitter or the transmitterand the receiver are distributed in the micro-cell, or where some or allof the cooperating terminals are idle, rather than actively engaged intheir own sessions. Indeed, advantage can be obtained in terms of areduced intra-cell communication requirement if the cooperatingterminals are otherwise idle, provided that there are sufficient of themin close proximity to the first mobile terminal. Examples of such casescan be found in appendix 1.

The optimum number of user terminals required to form a group representsa trade-off between the advantages of spatial processing gain andstatistical multiplexing gain derived from a multiplicity of users andthe bandwidth requirement for the exchange of information between themembers of the group or micro-cell, depending on the circumstances. Thiswill be determined by the system user such that the gain in spectralefficiency is close to the maximum value, such as that indicated in theFIGS. 6-11, which illustrate graphically the relationship betweenspectral efficiency and the number of terminals in a group. In addition,the spectral efficiency gain can be enhanced by control of the amount ofprocessing that is performed in the first processing step. Ideally, theterminals should exchange as much information as possible to facilitatethe second processing step to recover the signal. However, the bandwidthrequired for this information exchange is part of the overall spectrumresource, and therefore a choice should be made to balance the demandsof information exchange against the total bandwidth budget.

Spatial processing gain increases with the number of antennas at eachend of the link. However, the amount of spectrum required forcommunication between the cooperating terminals also increases with thenumber of terminals. An optimum occurs when the number of terminals inthe group forming the micro-cell is large enough to provide significantspatial processing gain but is not so large that the overall systemcapacity is degraded by the amount of spectrum required forcommunication between the terminals.

Furthermore, it is important to have a tight frequency re-use patternamongst the micro-cells. Adding a further mobile terminal to amicro-cell may be counterproductive if it results in an increase in thefrequency re-use factor for the micro-cellular ad-hoc network, forexample due to anomalous propagation conditions.

Not all terminals contribute to the trade-off between spatial processinggain and spectrum required for communication between terminals in thesame way.

Propagation effects such as shadowing can lead to a terminal seeing arelatively low signal power and hence being unable to make a significantcontribution to the final combined signal power. Variation inpropagation conditions between antenna elements is more likely when theyare disposed on different terminals than it is in a conventional MIMO(multiple input multiple output) system (i.e. one having multipleantennas at both the transmitter and at the receiver) in which theantenna elements are collocated on the same terminal.

The amount of spectrum required for communication between cooperatingterminals depends upon whether the terminals are actively engaged intheir own sessions or are idle. Active terminals require more spectrumthan idle terminals and hence the optimum number required to form amicro-cell is lower.

When selecting terminals for inclusion in a micro-cell it is importantboth that the terminals within the micro-cell can communicate well (i.e.good quality link) with each other and that each of the terminals cancommunicate well with the base station.

The criteria for selecting terminals for inclusion in a micro-cell canbe summarised as follows:

-   -   Contribution to the spatial processing gain    -   Terminal activity (active or idle terminal)    -   Channel bandwidth    -   Effect on the micro-cellular frequency re-use factor    -   Total spectral allocation

Two preferred techniques for determining which terminals should beselected for inclusion in a micro-cell are described below. It will beunderstood by those skilled in the art that there are other possibletechniques which could be used.

In the first technique, it is assumed that the channel bandwidth W, theoverall spectral allocation S, the intra-cell frequency allocation ω andthe micro-cell frequency re-use φ are known to the terminal, eitherpre-set or supplied by the network management system (NMS). Theprinciples of the algorithm can be illustrated by considering thedownlink example used above in which a base station with multipleantennas is transmitting to a first mobile terminal, which is in amicro-cell with other similar terminals. It is assumed that the numberof transmit antennas is known, as is the required signal to noise ratiofor the base station to mobile link and the mobile terminal to mobileterminal links. The last of these three parameters is required because aterminal, in deciding which other terminals should be within amicro-cell needs to ensure that it can communicate sufficiently wellwith the other terminals within the micro-cell (as in stage 1 below).

The first mobile terminal seeks other mobile terminals according to thefollowing algorithm:

1. Determine that the signal to noise ratio between cooperating terminaland the first mobile terminal can be met for intra-micro-cellcommunication within the constraints of the micro-cellular frequencyre-use φ. The necessary information may be available from the physicallayer of the ad-hoc network protocol performing the inter-terminal (i.e.intra-micro-cell) communication: for example, the mobile terminals couldmonitor received signal strength (RSSI) or bit error ratio (BER) on thelink.

2. Select idle terminals in preference to active terminals. Idleterminals are preferred to active terminals because theirintra-micro-cell communication requirements are lower but there may notbe enough idle terminals or the micro-cellular re-use condition may meanthat some idle terminals cannot be used. For example, interferencebetween micro-cells, due to anomalous propagation, may prevent amicro-cell from being fully populated with idle terminals whilemaintaining the necessary signal to noise ratio and frequency re-usefactor. Hence, it may sometimes be advantageous to make use of activeterminals that are located in the vicinity of the first mobile terminal.

3. Maximise the spatial processing gain by increasing the number ofcooperating terminals and selecting terminals that make the greatestcontribution to the overall combined signal power. In this step, it isthe quality of the base station to terminal link which is important.Algorithms for doing this based on the characteristics of the combinedsignal, such as an error check, final BER, or eigenvalue of the MIMOsignal, have been described by Smith et al in U.S. patent applicationSer. Nos. 10/083,094 and 10/083,100. However, such techniques as theydescribe, which rely on the combined signal, may not be appropriate herebecause of the increased load they place on the intra-micro-cellcommunication, due to the need to provide signals from all of the mobileterminals in the vicinity of the first mobile terminal. An alternativemeasure, such as RSSI, is preferred because this can be performed ateach mobile terminal without communication between terminals and theterminal can be selected or not for inclusion in the micro-cell of thefirst mobile terminal based on a local measurement of this parameter.

In the second preferred technique, it is assumed that in practice it islikely that the total spectrum S and the spectrum ω allocated tointer-terminal communication will be fixed. From these values and thechannel bandwidth W, an estimate of the optimum number of antennas canbe pre-determined, for example by solving equation 11. The choice of ωwill have been made assuming a frequency re-use factor that the ad-hocnetwork is capable of meeting at the required signal to noise ratio.Hence the only variable is to choose which of the terminals in thevicinity are to be included in the cooperating group. In the firstinstance, the cooperating group should be chosen from idle terminals.Active terminals should only be included if there are insufficient idleterminals. The group should be expanded until ω is fully utilised. Asearch should be continually made for a terminal with a better RSSI thanthe lowest in the group, and if one is found it should replace the onewith the worst RSSI in the micro-cell, particularly if the one with thebetter RSSI is an idle terminal.

It will be understood that the above description of a preferredembodiment is given by way of example only and that variousmodifications may be made by those skilled in the art without departingfrom the spirit and scope of the invention.

1-4. (canceled)
 5. A method for operating a first mobile terminal of agroup of mobile terminals, the method comprising: at a first mobileterminal: generating a first symbol sequence; receiving at least asecond symbol sequence from a second mobile terminal of the group;generating a transmit signal based at least on the first symbol sequenceand the second symbol sequence using code division multiplexing;transmitting the transmit signal to a communication station, wherein thecommunication station includes a plurality of antennas.
 6. The method ofclaim 5, wherein the second mobile terminal is configured to transmitthe second symbol data sequence to the first mobile terminal over afirst frequency range, wherein transmissions between the communicationstation and the group of mobile terminals occurs outside the firstfrequency range.
 7. The method of claim 5, wherein the first mobileterminal includes a first antenna and a second antenna, wherein thefirst antenna is used to receive said second symbol sequence from thesecond mobile terminal, wherein the second antenna is used to transmitthe transmit signal to the communication station.
 8. The method of claim5, wherein the communication station is a base station.
 9. The method ofclaim 5, wherein the communication station is a mobile terminalbelonging to a second group of mobile terminals, distinct from saidgroup of mobile terminals.
 10. The method of claim 5, wherein the firstmobile terminal includes only one antenna.
 11. The method of claim 5,wherein the group of mobile terminals is formed from a superset ofmobile terminals based on measurements of signal strength oftransmissions between the mobile terminals of the superset.
 12. A firstmobile terminal comprising: a processor, and memory storing programinstructions, wherein the program instructions, when executed by theprocessor, cause the processor to implement: generating a first symbolsequence, wherein the first mobile terminal belongs to a group of mobileterminals; receiving at least a second symbol sequence from a secondmobile terminal of the group; generating a transmit signal based atleast on the first symbol sequence and the second symbol sequence usingcode division multiplexing; transmitting the transmit signal to acommunication station, wherein the communication station includes aplurality of antennas.
 13. The first mobile terminal of claim 12,wherein the first mobile terminal is configured to receive the secondsymbol data sequence over a first frequency range, wherein the basestation is configured to communicate with the group of mobile terminalsoutside the first frequency range.
 14. The first mobile terminal ofclaim 12, wherein the first mobile terminal includes a first antenna anda second antenna, wherein first mobile terminal is configured to receivesaid second symbol sequence using the first antenna, wherein the firstmobile terminal is configured to transmit the transmit signal to thecommunication station using the second antenna.
 15. The first mobileterminal of claim 12, wherein the communication station is a basestation.
 16. The first mobile terminal of claim 12, wherein thecommunication station is a mobile terminal belonging to a second groupof mobile terminals, distinct from said group of mobile terminals. 17.The first mobile terminal of claim 12, wherein the first mobile terminalincludes only one antenna.
 18. The first mobile terminal of claim 12,wherein the group of mobile terminals is formed from a superset ofmobile terminals based on measurements of signal strength oftransmissions between the mobile terminals of the superset.
 19. Anon-transitory memory medium for operating a first mobile terminal,wherein the memory stores program instructions, wherein the programinstructions, when executed by a processor, cause the processor toimplement: generating a first symbol sequence, wherein the first mobileterminal belongs to a group of mobile terminals; receiving at least asecond symbol sequence from a second mobile terminal of the group;generating a transmit signal based at least on the first symbol sequenceand the second symbol sequence using code division multiplexing;transmitting the transmit signal to a communication station, wherein thecommunication station includes a plurality of antennas.
 20. Thenon-transitory memory medium of claim 19, wherein the first mobileterminal is configured to receive the second symbol data sequence over afirst frequency range, wherein the base station is configured tocommunicate with the group of mobile terminals outside the firstfrequency range.
 21. The non-transitory memory medium of claim 19,wherein the first mobile terminal includes a first antenna and a secondantenna, wherein first mobile terminal is configured to receive saidsecond symbol sequence using the first antenna, wherein the first mobileterminal is configured to transmit the transmit signal to thecommunication station using the second antenna.
 22. The non-transitorymemory medium of claim 19, wherein the communication station is a basestation.
 23. The non-transitory memory medium of claim 19, wherein thecommunication station is a mobile terminal belonging to a second groupof mobile terminals, distinct from said group of mobile terminals. 24.The non-transitory memory medium of claim 19, wherein the group ofmobile terminals is formed from a superset of mobile terminals based onmeasurements of signal strength of transmissions between the mobileterminals of the superset.